The present invention generally relates to surface-mount electronic devices. More particularly, this invention relates to a method of forming solder bumps on surface-mount devices using a solder jetting device.
Flip chips, ball grid arrays (BGAs), chip scale packages (CSPs), chip resistors and chip capacitors are examples of surface-mount devices, i.e., discrete circuit devices mounted to the surface of a substrate, such as a printed circuit board (PCB), printed wiring board (PWB), flexible circuit, or a silicon, ceramic or insulated metal substrate. These devices rely on solder connections to both secure the chip to a substrate and electrically interconnect the device to conductors formed on the substrate. The size of a flip chip is generally on the order of a few millimeters per side, while chip capacitors and resistors are typically smaller. As a result, the conductors required for surface-mount devices are narrow, e.g., line widths of about 0.5 millimeter or less, and typically spaced apart about 0.5 millimeter or less. The conductors must be formed of a solderable material, which as used herein means that a tin, lead or indium-based solder alloy is able to adhere to the conductor through the formation of a strong metallurgical bond.
Because of the small size of the solder connections, soldering a surface-mount device to its conductor pattern requires a significant degree of precision. Reflow solder techniques are widely employed for this purpose, and typically entail precisely depositing a controlled quantity of solder using methods such as printing and electrodeposition. When mounting flip chips, BGAs, CSPs and other larger surface-mount device chips, solder is typically deposited on bond pads on the chip and then heated above its liquidus temperature to yield spherically-shaped solder xe2x80x9cbumps.xe2x80x9d After cooling to solidify the solder bumps, the chip is soldered to the conductor pattern by registering the solder bumps with their respective conductors and then reflowing the solder to form spherical-shaped solder connections. Solder bump size, chip placement and solder reflow must be precisely controlled to produce solder connections having adequate height and appropriate current-carrying and thermal management capabilities. As is well known in the art, solder connection height must often be controlled to prevent the surface tension of the molten solder bumps from drawing the chip excessively close to the substrate during the reflow operation. Sufficient spacing between a chip and its substrate, which may be termed the xe2x80x9cstand-off height,xe2x80x9d is desirable for enabling stress relief during thermal cycles, allowing penetration of cleaning solutions for removing undesirable processing residues, and enabling the penetration of mechanical bonding and underfill materials between the chip and its substrate.
Solder connection height is controlled in part by limiting the surface area of the solderable bond pads on which the solder bumps are formed, and by controlling the amount of solder deposited on the bond pads. Solder jetting is a known technique for accurately placing solder material and controlling the volume of solder material deposited on a bond pad. Jetting techniques accurately deposit molten droplets of solder on a wettable surface. Current jetting techniques are limited to relatively low-volume solder droplets, generally diameters of about 105 Fm or less. The coordinates of each bond pad are established, and then one or more solder droplets are jetted onto each bond pad to achieve the desired solder bump volume. After jetting, the bond pad is fluxed and the solder droplets reflowed to form a single coalesced solder bump on each bond pad.
A limitation of solder jetting methods is the size of the solder bumps that can be reliably produced. The size of solder droplets produced by current jetting apparatuses is generally insufficient for larger chips such as BGAs, CSPs and many flip chips in terms of stand-off height, current capacity and thermal management issues. Accordingly, it would be desirable if a method were available for producing relatively large solder bumps whose volume and subsequent solder connection height are controlled by a solder jetting technique.
The present invention provides a method for forming solder bumps and solder connections for surface-mount devices. The invention utilizes a solder jetting technique by which a controlled amount of solder is accurately deposited to yield solder bumps on the bond pads of a device, which upon reflow form solder connections having sufficient stand-off height to promote stress relief during thermal cycling, and achieve the necessary electrical and thermal capabilities required of the solder connections.
The method of this invention generally entails solder jetting multiple solder droplets onto a surface to form a two-dimensional base pattern on a bond pad of a surface-mount device. An additional solder droplet is then solder jetted onto the xe2x80x9cbasexe2x80x9d droplets of the two-dimensional base pattern so that the additional droplet contacts at least two of the base droplets and yields a three-dimensional structure overlying the bond pad. All of the solder droplets are then reflowed to form a single coalesced solder bump on the bond pad. To maximize solder volume for a given bond pad, the base droplets are preferably located at or sufficiently near the perimeter of the bond pad so that surface tension will cause the molten solder to coalesce on the bond pad during reflow. The additional droplet is deposited to contact at least two and preferably all of the base droplets, which may be spaced so that the additional droplet is supported above the bond pad, or jetted down among the base droplets so as to contact the bond pad.
In view of the above, the present invention enables the use of a solder jetting technique to produce a three-dimensional structure of solder droplets, which can then be reflowed to form a relatively large solder bump on a bond pad. Three-dimensional structures produced by the method of this invention are stable, which allows the device to be handled prior to reflow. More particularly, the solder droplets used to form the two-dimensional base pattern define a pocket in which the additional droplet is securely received. With this invention, an appropriate volume of solder can be deposited to produce solder bumps that will form a solder connection having an adequate height to allow an underfill material to flow beneath the device, promote stress relief during thermal cycling, and achieve the necessary electrical and thermal capabilities required of the solder connections.
Other objects and advantages of this invention will be better appreciated from the following detailed description.